1. Field of the Invention
This servo mount design is related to the specific connection between the device or arrangement for controlling an aircraft and a controlled element.
2. Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98
It is often necessary or desirable to power the motion of an aircraft control surface with an electro-mechanical servo. The most common use of servo powered flight controls is in the radio controlled aircraft hobby industry. There is also a rapidly growing market for commercial and military remotely piloted vehicles, which also employ the use of servos to power their flight controls. The latest trend in full scale aviation is all electric flight control actuation, or “fly by wire”. It is conceivable that servo powered flight controls will replace the hydro-mechanical powered systems predominantly used in manned aircraft today, since they are very compatible with the fly by wire concept. There are a large variety of commercially available servos which provide an output via a rotating shaft. In all previous endeavors, the servo is mounted to the airframe in a location other than on the hinge line of the control surface it powers, and the servo shaft drives the control surface through a linkage in one or a combination of the following manners:                (a) The servo shaft is fitted with a lever arm (servo arm), which is connected to a lever arm on the control surface through a mechanical link. The link is rigid enough to transfer both push and pull forces between the two arms, thereby transferring the rotational motion of the servo shaft to the control surface.        (b) The servo shaft is fitted with two parallel and opposing lever arms, 180 degrees apart, which are connected to matching double lever arms on the control surface via two pull cables. In this way each cable transfers a pull force from the servo arm to which it is attached to the respective control surface arm, thereby transferring the rotational motion of the servo shaft to the control surface. Each cable transfers motion in only one direction.        (c) The servo shaft is fitted with a pulley, which is connected to a corresponding pulley on the control surface through pull cables. As in (2) above, the cables serve to transfer the rotational motion of the servo shaft to the control surface.        
The above methods of driving a control surface involve a conversion of rotational motion at the servo shaft to linear motion, which is then reconverted back to rotational motion at the control. There are several inherent problems with these linkage drive systems:                (a) A linkage generates a linear load which must be absorbed by both the servo shaft bearings and the control surface hinges. The linear load is at its greatest when the control is fully deflected, which is also the point when the air loads on the control hinges are highest. Both these loads are additive on the hinges. This load accelerates wear on the servo bearings and control hinges, thereby increasing the need for periodic maintenance and part replacement.        (b) At large control surface deflections, the geometry of a link or cable becomes severe.                    (1 ) The effective lengths of the lever arms become shorter at high control deflections when the air loads are strongest. This results in an exponentially increasing load on the servo and the control hinge. To reduce compliance in the structure due to these loads, the control drive assembly must be strengthened significantly. The result is added weight, which has negative aircraft performance consequences.            (2) At the extremes of control travel, linkage angularity results in non-linear control deflection rates. That is, the control moves at a different angular rate than the angular rate of the servo output shaft. This requires complicated electronic or mechanical compensation to return control movement to a proportionally linear motion.                        (c) Friction is generated at the connection points of a link, the servo bearings, and the control hinge. Friction subtracts from the servo power that reaches the control surface. To compensate, a stronger servo is required, which again means more weight to the aircraft.        (d) A linkage must have some tolerance at the connecting points to ensure it is free to pivot. As the connections wear, the tolerance naturally increases. Additionally, when force is applied through any mechanical assembly, there is deflection due to stress. In a cable linkage, the cables stretch significantly under load. The end result of these factors is lost motion between the servo and the control surface. Lost motion is detrimental for two reasons:                    (1) Lost motion allows air loads to deflect the control surface from the commanded position. This reduces the precision control of the surface, thereby reducing effective control of the aircraft.            (2) Lost motion reduces the overall stiffness of the control assembly, which increases the risk of control surface flutter. Flutter is potentially catastrophic to the structural integrity of an aircraft.                        (e) By their nature, linkage elements located at the control surface introduce mass behind the hinge line. This increases the weight of the surface, thereby increasing the tendency of the surface to flutter. Larger counterweights must be used to compensate for the weight of the control drive component, thereby adding weight to the aircraft.        
Even with the mentioned shortcomings, linkages were necessary in past endeavors because:                (a) Early generation servos were underpowered, which required a properly designed linkage to increase the mechanical advantage of the servo over the control surface.        (b) Early generation servos had limited adjustment capability and no ability to change the direction of shaft motion in relation to an input. An adjustable linkage was necessary to center the control surface at the neutral point and/or to reverse the direction of control motion in relation to servo shaft motion.        (c) A few years ago, it was accepted that control surface travel should be limited to less than 30 degrees from the center position for aerodynamic reasons. In the past, commercially available servos had a range of travel in excess of 45 degrees from center with no limit of travel adjustment, which made a motion reduction linkage necessary to reduce the range of travel to accepted limits at the control surface.        
In recent years, however, there have been numerous advances in both servo and aircraft design, so we are no longer bound by the limitations of just a few years ago. It has been shown that there is performance to be gained from using control travel in excess of 45 degrees. Electronic servo controls now make it possible to adjust the travel limit, set the centering point, and change the direction of rotation of the servo shaft without the need for cumbersome and restrictive mechanical linkages to do the same. Improved motor technology has provided servos that are now strong enough to drive a control surface without the need for mechanical advantage. Linkage ratios on servo driven flight controls are frequently reduced to 1:1. The traditional reasons for using a linkage between a servo and the control it drives no longer apply. It is now possible to drive an aircraft control surface directly from the servo shaft, without intermediary mechanisms to modify the servo output.
This method of directly driving the control surface has still not been used since current servo case design impedes doing so. Commercially available servos share a common generic mounting method in which tabs are provided on each end of the case to mount the servo using two or more screws to a fixed position on the airframe. This tab system provides flexibility in mounting the servo in the greatest number of standard applications, all of which involve the use of linkages to transfer the servo motion to the control. Though providing flexibility, the resulting design of the modern commercial servo is not at all suited to being mounted on the hinge line of a control surface.
The output shaft is near the middle of the servo body, which is an undesirable location for hinge line mounting as it causes the servo case to protrude significantly into the area occupied by the control surface. The provided mounting tabs extend further into the control surface space, further obstructing control motion. Because a large portion of the servo body, including half of the mounting tabs, is located past the hinge line, there is no obvious way to secure the servo to the aircraft. The mounting tabs are placed in an inconvenient, normally inaccessible location, and are at an awkward angle when securing the servo at the hinge line. In short, the standard shape and design of commercially available servos has precluded them from even being considered for hinge line mounting. Therefore, direct servo drive of an aircraft control surface is not an obvious or inevitable evolution of prior art.